Many physical implementations of quantum computers have been proposed. Knill, Laflamme, and Milburn, Nature 409, 26 (2001), for example, proposed a quantum computing system using optical qubits manipulated with linear optics. Alternatively, optical quantum information systems can use the “cluster state model” of quantum computation introduced by R. Raussendorf and H. Briegel, Phys. Rev. Lett. 86, 5188 (2001). The cluster state model requires the generation of a specific entangled state (or the cluster state), augmented by single qubit measurements in arbitrary bases, or, equivalently, arbitrary single qubit operations together with single-qubit measurements in a particular basis. Linear optical quantum computing (LOQC) systems can eliminate the need for a direct interaction between photonic qubits by using measurement-induced nonlinearities, but generally require a quantum memory for photons, which may be difficult to implement.
Some other particularly promising proposals for quantum computing systems implement unitary operations and readout in matter qubits via laser-driven optical transitions. Examples of these systems include ion-trap systems such as described by J. Cirac and P. Zoller, Phys. Rev. Lett. 74, 4091 (1995), system using nitrogen-vacancy (N-V) defects in diamond as described by Jelezko et al., Phys. Rev. Lett. 92, 076401 (2004), and systems using the Pauli-blockade effect in quantum dots with a single excess electron as described by Pazy et al., Europhys. Lett 62, 175 (2003) or by Nazir et al., Phys. Rev. Lett. 93, 150502 (2004). However, many of the systems using optical addressing or readout of matter qubits also use a relatively short-range interaction (e.g., the Coulomb interaction) in two-qubit gates. The short effective range of the interactions requires that the interacting matter qubits be close together. In contrast, optical addressing of the qubits requires that the separations between matter qubits be large enough to that the optical pulses can resolve the qubits. The short-range interaction for 2-qubit operations and the optical addressing of the qubits therefore seem to lead to contradictory system requirements. (Quantum computing systems implemented using ion traps can avoid this contradiction but must use mini-trap arrays or moving qubits to obtain scalability.)
Systems and methods for efficiently producing remote interaction of stationary qubits, for example, for high-fidelity entanglement of static qubits are thus sought.